The four-winged early birds had been identified from fossilised remains a number of years ago, but it was unclear whether the creatures were precursors to modern birds or whether they represented an evolutionary cul-de-sac and had simply died out.

However, eleven skeletons of primitive birds discovered at the Shandong Tianyu Museum of Nature feature evidence of having large feathers on their hind limbs. The remains date from the early Cretaceous period (around 120 million years ago) and, according to the study, "provide solid evidence for the existence of enlarged leg feathers on a variety of basal birds".

Today's two-winged situation could then be the result of a gradual reduction in feathering of these hind limbs, probably as a result of the birds living on the ground and needing to walk around unencumbered.

"If an animal has big feathers on its legs and feet, it's definitely something that's not good for fast running," said Xing Xu from Linyi University in Shandong province in an interview with New Scientist.

The fossil finds help bolster the case for four-winged early birds, however the evidence is not definitive. As a result, Xu and his fellow researchers intend to look to other remains in the museum's collection as well as investigating whether the feathers and wings would have been capable of flight.

I wonder what evolutionary reason why birds never developed an additional pair of forearms, seems like an advantage to be able to manipulate things with limbs, walk/run, and fly.

Just too late in the process after 4 limbs already became the default?

In general, I'd say yes. An extra set of limbs would be such a major change for a (derived) tetrapod, that it would probably require too many simultaneous changes to be likely (while producing a viable organism). Way too much regulatory stuff you'd have to change. We're not like an annelid or an arthropod you could just add another segment to.

It's also not really clear it would provide an advantage. Birds get along quite well as it is.

I wonder what evolutionary reason why birds never developed an additional pair of forearms, seems like an advantage to be able to manipulate things with limbs, walk/run, and fly.

Just too late in the process after 4 limbs already became the default?

Tetrapod's do not have anything to evolve a 2nd set of limbs from in the timespan from then to now. Cephalopod's would be a more likely candidate for winged creatures with extra limbs. This is of coarse ignoring all the Arthropod's that do have wings and lots of limbs.

I wonder what evolutionary reason why birds never developed an additional pair of forearms, seems like an advantage to be able to manipulate things with limbs, walk/run, and fly.

Just too late in the process after 4 limbs already became the default?

As far as I know, all vertebrates have 4 limbs, with the wings in birds being the equivalent of arms/forelegs in other vertebrates. There's simply nowhere else in our common (on a very basic level) skeletal structure for extra limbs to attach to.

The four winged planform is very efficient at generating lift when not much power is available. Microraptor did not have a wishbone, for example. It makes for slow, low stall speed, sustainable flight. There's a lot of drag, but also a lot of lift.

We did the same in early aviation, when we had 75 hp engines weighing a metric tonne. In Microraptor's case, the forewings were for control of direction and to provide primary lift, while the rear wings (legs) were providing the thrust as microraptor's skeleton shows it had much stronger legs than arms.

I wonder what evolutionary reason why birds never developed an additional pair of forearms, seems like an advantage to be able to manipulate things with limbs, walk/run, and fly.

Just too late in the process after 4 limbs already became the default?

Evolution really does not work that way. It is not evolving towards anything. Mutations are completely random. Those that are beneficial towards permitting increased breeding potential tend to stick around, those that are detrimental or neutral tend not to. To acquire more limbs would require a massive series of mutations over millions of years with each of them becoming beneficial along the way. Chances are very small of that occurring.

The lack of centaur-birds (bird-taurs?) disappoints. But still, interesting. If they found about a dozen fossilized remains of these creatures, how many would that imply existed? How long would they have been around?

I wonder what evolutionary reason why birds never developed an additional pair of forearms, seems like an advantage to be able to manipulate things with limbs, walk/run, and fly.

Just too late in the process after 4 limbs already became the default?

Evolution really does not work that way. It is not evolving towards anything. Mutations are completely random. Those that are beneficial towards permitting increased breeding potential tend to stick around, those that are detrimental or neutral tend not to. To acquire more limbs would require a massive series of mutations over millions of years with each of them becoming beneficial along the way. Chances are very small of that occurring.

And remember, when it comes to the likelihood of survival, half of all mutants are below average.

This. Now we must question the causal relationship between earliness and worms, as this study suggests improved thrust and lift may well have been the deciding factor. Mothers everywhere will have to prepare for the rejoinder "yes, but they also had four wings . . .."

I wonder what evolutionary reason why birds never developed an additional pair of forearms, seems like an advantage to be able to manipulate things with limbs, walk/run, and fly.

Just too late in the process after 4 limbs already became the default?

yeah I wondered that...but the answer is obvious..and it is what you said. four limbs was the norm for all the descendants of the original tetrapods and apparently it was evolutionarily too unlikely/difficult to evolve extra limbs. Just like most mammals are stuck with 7 cervical vertebrae, and for various reasons it is very difficult to evolve out of that configuration (mutations that change number of cervical vertebrae in mammals tend to cause a high risk of cancer, that's what I read in NewScientist once upon a time)..so mammals mostly could not evolve out of that configuration. Birds, however, do not have this probabilistic "blockage" in their genetics and have a wide variety of different cervical vertebrae numbers.

It is probably just extremely unlikely for evolution to find a way to successfully add limbs to tetrapods in ways that would persist and give advantage. Such is the genetic configuration...it has probably "painted itself into a corner" so to speak.

I wonder what evolutionary reason why birds never developed an additional pair of forearms, seems like an advantage to be able to manipulate things with limbs, walk/run, and fly.

Just too late in the process after 4 limbs already became the default?

Evolution really does not work that way. It is not evolving towards anything. Mutations are completely random. Those that are beneficial towards permitting increased breeding potential tend to stick around, those that are detrimental or neutral tend not to. To acquire more limbs would require a massive series of mutations over millions of years with each of them becoming beneficial along the way. Chances are very small of that occurring.

I have to disagree. the fact that birds can easily add vertebrae and mammals can't (in evolutionary terms), and the fact that conditions like polydactyly can arise out of simple mutations would argue that adding extra iterations of things is not necessarily such a massive evolutionary undertaking. And just like (and oppsite to) how sea mammals and snakes have degenerate or disappeared limbs resulting from a mutation that initially reduced certain limbs, conceivably an extra limb from a simple mutation might be able to hang around. Birds evolved feathers from mutated scute homologues that eventually lead to very complex structures. A simple replication that offers some advantage might stick around, if it is likely to happen and not give you cancer or something.

I don't think things are as simple as you argue. The difference between mammals and birds in flexibility of cervical vertebrae numbers might be more instructive in this case. Which means that...his simple statement that "4 limbs had become the default and it was too late to change" might well be close to the truth in some sense.

Extra limbs might be a good "idea" (adaptation), but evolution cannot not find a path to them. Either the limb increase mutations cause harmful side-effects (genetic configuration has "painted itself into corner"), or the evolutionary pathway in terms of useful intermediary structures is just too unlikely.

Alright. I am going to say it. To hell with all of this scientific rambling about skeletal structure and whether it would work or not. I want to know if they taste good. This story means twice the hotwings.

Alright. I am going to say it. To hell with all of this scientific rambling about skeletal structure and whether it would work or not. I want to know if they taste good. This story means twice the hotwings.

I wonder what evolutionary reason why birds never developed an additional pair of forearms, seems like an advantage to be able to manipulate things with limbs, walk/run, and fly.

Just too late in the process after 4 limbs already became the default?

Pretty much, yes. Evolution reuses parts. Unless there's a simple mechanism to turn on a gene or two that would result in an entire extra set of limbs, it won't happen. We see quick evolutionary progress in some areas because that does happen. But if there isn't a mechanism for a specific thing, it won't happen.

I don't agree with this four wing hypothesis, because the movements of legs are very different from arms, or forelegs. The motions are too different. To say that because hind legs had large feathers and were, therefor, wings, is too great a statement for me to accept. More likely, those hind legs were being used for stabilization during flight, and for use as a rudder during turns, as tails are used .

I wonder what evolutionary reason why birds never developed an additional pair of forearms, seems like an advantage to be able to manipulate things with limbs, walk/run, and fly.

Just too late in the process after 4 limbs already became the default?

Evolution really does not work that way. It is not evolving towards anything. Mutations are completely random. Those that are beneficial towards permitting increased breeding potential tend to stick around, those that are detrimental or neutral tend not to. To acquire more limbs would require a massive series of mutations over millions of years with each of them becoming beneficial along the way. Chances are very small of that occurring.

And remember, when it comes to the likelihood of survival, half of all mutants are below average.

Don't you mean, that more than half of mutations are below average?Essentially random changes to something that "works" are more likely to make things "worse", than make them "better".

I wonder what evolutionary reason why birds never developed an additional pair of forearms, seems like an advantage to be able to manipulate things with limbs, walk/run, and fly.

Just too late in the process after 4 limbs already became the default?

Evolution really does not work that way. It is not evolving towards anything. Mutations are completely random. Those that are beneficial towards permitting increased breeding potential tend to stick around, those that are detrimental or neutral tend not to. To acquire more limbs would require a massive series of mutations over millions of years with each of them becoming beneficial along the way. Chances are very small of that occurring.

I have to disagree. the fact that birds can easily add vertebrae and mammals can't (in evolutionary terms), and the fact that conditions like polydactyly can arise out of simple mutations would argue that adding extra iterations of things is not necessarily such a massive evolutionary undertaking. And just like (and oppsite to) how sea mammals and snakes have degenerate or disappeared limbs resulting from a mutation that initially reduced certain limbs, conceivably an extra limb from a simple mutation might be able to hang around. Birds evolved feathers from mutated scute homologues that eventually lead to very complex structures. A simple replication that offers some advantage might stick around, if it is likely to happen and not give you cancer or something.

I don't think things are as simple as you argue. The difference between mammals and birds in flexibility of cervical vertebrae numbers might be more instructive in this case. Which means that...his simple statement that "4 limbs had become the default and it was too late to change" might well be close to the truth in some sense.

Extra limbs might be a good "idea" (adaptation), but evolution cannot not find a path to them. Either the limb increase mutations cause harmful side-effects (genetic configuration has "painted itself into corner"), or the evolutionary pathway in terms of useful intermediary structures is just too unlikely.

My point was mostly about intent. There is not some ultimate goal of evolution nor are there 'higher evolved' life forms. Evolution is incredibly inefficient and makes millions more mistakes than successes. And is incredibly slow and gradual over extremely long periods of time.

How do you 'find' something in a museum? Hasn't it, almost by definition, already been found if it's in a musuem?

Museums often have a huge backlog of specimens, sometimes going back hundreds of years, that haven't been scrutinized heavily yet. Think of "finding" an unopened Christmas present in your attic that you'd forgotten was there.

I find it really interesting that a lot of digging nowadays occurs in museum cabinets, probably among rows and rows of dusty shelves like something out of Indiana Jones

If only there were a global fossil catalog with visible light and X-ray images and 3D models... like a Google Books for fossils. Researchers from all over the world could easily search through the catalog to find similar fossils.

Early Birds and Bird-like relatives have a huge variations in wing arrangements. We've known of four winged (Or atleast four feathered limbed) organisms in the Clade Aves since the discover of Archaoptryx in the 1800s (archaeopteryx was a feather Archosaur though, unrelated to modern birds).

What's rather interesting is that at the time of the K-Pg event there were multiple clades of Birds, including a number of species that were rear winged or had radically different wing configurations. Those birds we are left with are just a small handful of the full diversity of Avian types that have ever lived.

BTW: the word Clade is wonderful, it refers to any group of species or individuals sharing a common ancestor (consisting of that ancestor and all its decedents). If you can't be bothered to remember King Phillip..., just call it a Clade. You'll be semantically correct.

Are we even sure the fossils are real? It wouldn't be the first time someone displayed fake remains of fictional creatures.

Not often displayed, but sure there are attempts by finders to 'raise the value' of them by gluing pieces together, it is googeable. IIRC the original Microraptor (?) was found such, which delayed its interpretation et cetera (and probably damaged some details).

Museums catch most of those though, and 11 specimens isn't easy to glue together. Besides, it sounds like they weren't displayed but found boxed.

Garst wrote:

Isn't there even a skeleton of a "dragon" in a museum somewhere in England?

Claim in need of reference.

AlStar wrote:

Quote:

fossils found in a Chinese museum.

How do you 'find' something in a museum? Hasn't it, almost by definition, already been found if it's in a musuem?

That is perhaps the only thing in the first Indiana Jones movie that makes sense. Do you know how these things gets boxed, shelved, unboxed, hopefully catalogized, boxed, shelved, unboxed, hopefully restored, hopefully identified yet again, boxed, shelved, unboxed, perhaps displayed, et cetera ad infinitum? Me neither, but I hear it is something like that.

I also seem to remember descriptions of some specimen has taken decades before they get a proper identification. Usually they have gotten lost (misplaced et cetera), but anyway...

Adding or removing limbs should in principle be simple, we were once segmented like arthropods as our backbone and rib cage (et many cetera) tells us, and limb formation is like toes a bud development process. ibad mentions that it happens, even though additional placement is most often close to were the original appendage would be placed.

But tetrapods have never done that, akin to how we have never added extra toes. Removal has happened a lot. Arthropods are much less constrained (proving the ability), but arachnids behaves like tetrapods. Et cetera, showing that it is something for biologists to bite into, and as far as I know they don't yet know either.

fjpoblam wrote:

And remember, when it comes to the likelihood of survival, half of all mutants are below average.

You mean that of the sum of variation, half of it is by definition below average. So that is a fact. =D

But mutations are but a small part of natural variation, due to gene expression and development (phenotype is not genotype).

In fact, according to near neutral drift theory and molecular biology both, most mutations in medium sized populations are too small to be noticeable for selection. Even if you grow the population size to near infinity, so selection in principle can pick up on minuscule fitness differences (but still is in the hands of contingency that can make an allele go extinct), many or most mutations will be exactly neutral. (Change of base pair but the same amino acid; SNPs (base pair change) is the most common mutation, and the genetic code is hugely redundant.)

Of the rest, I believe most are lethal. This is part of the explanation why some (IIRC) 70 % of fetuses are rejected at the zygote stage.

Even slightly negative mutations can be rather visible and happens too often for comfort. Say, increasing cancer frequency decrease fitness.

Slightly positive mutations are rare, but observed. Examples are the mutations for lactose tolerance seen in many human subpopulations, of which there are some 5 or so types known. Now add millions of years (lactose tolerance took some 1000s of years after the start of keeping animals for milk), and you may get the sum of small positive changes to add to something huge. Say, getting rid of those pesky wisdom teeth that many have no room for anyway.

I wonder what evolutionary reason why birds never developed an additional pair of forearms, seems like an advantage to be able to manipulate things with limbs, walk/run, and fly.

Just too late in the process after 4 limbs already became the default?

Additional limbs. Additional drag.

Between their prehensile feet and their beaks, birds have no need for an extra pair of limbs to manipulate things.

Beside, look at the vertebrate lineage: no species has ever had more than 4 limbs. It's a very basic element of vertebrate embryonic development. The evidence points to there being no evolutionary way to get there from here or anywhere else in the vertebrate tree.

Slightly positive mutations are rare, but observed. Examples are the mutations for lactose tolerance seen in many human subpopulations, of which there are some 5 or so types known. Now add millions of years (lactose tolerance took some 1000s of years after the start of keeping animals for milk), and you may get the sum of small positive changes to add to something huge. Say, getting rid of those pesky wisdom teeth that many have no room for anyway.

My family already has those genes and is busily rolling them out to the rest of the human population. We estimate it will take somewhere between 10,000 to 50,000 years for the rest of you to get the upgrade. It's a dominant gene and results in a random (but less than standard) number of adult molars. We're hoping to find another useful allele that will interact in a nice way and determine the number of adult teeth at 28. (We only has 26!) Unfortunately, we doesn't know what happens when a person is homozygous for the allele. Maybe no teethes!

Slightly positive mutations are rare, but observed. Examples are the mutations for lactose tolerance seen in many human subpopulations, of which there are some 5 or so types known. Now add millions of years (lactose tolerance took some 1000s of years after the start of keeping animals for milk), and you may get the sum of small positive changes to add to something huge. Say, getting rid of those pesky wisdom teeth that many have no room for anyway.

My family already has those genes and is busily rolling them out to the rest of the human population. We estimate it will take somewhere between 10,000 to 50,000 years for the rest of you to get the upgrade. It's a dominant gene and results in a random (but less than standard) number of adult molars. We're hoping to find another useful allele that will interact in a nice way and determine the number of adult teeth at 28. (We only has 26!) Unfortunately, we doesn't know what happens when a person is homozygous for the allele. Maybe no teethes!

I wonder what evolutionary reason why birds never developed an additional pair of forearms, seems like an advantage to be able to manipulate things with limbs, walk/run, and fly.

Just too late in the process after 4 limbs already became the default?

As far as I know, all vertebrates have 4 limbs, with the wings in birds being the equivalent of arms/forelegs in other vertebrates. There's simply nowhere else in our common (on a very basic level) skeletal structure for extra limbs to attach to.